Discovery of a Thermodynamic-Control Two-Dimensional Cs 6 Pb 5 I 16 Perovskite with a Unique Green Emission Color via Dynamic Structural Transformation.

New perovskite materials of two-dimensional (2D) all-inorganic Ruddlesden-Popper (RP) perovskite Cs 6 Pb 5 I 16 nanosheets were successfully obtained from the structural transformation of 2D PR-phase Cs 7 Pb 6 I 19 nanosheets. The 2D RP-phase Cs 6 Pb 5 I 16 perovskite nanosheets exhibited unique green emission with an emission wavelength of ∼500 nm. The crystal structure of the 2D RP-phase Cs 6 Pb 5 I 16 perovskite nanosheets was determined by powder X-ray diffraction (XRD), high-resolution transmission electron microscopy, and atomic force microscopy. The time-dependent photoluminescence measurements and XRD spectra were used to observe the optical and structure transformations from 2D Cs 7 Pb 6 I 19 ( n = 6) to 2D Cs 6 Pb 5 I 16 ( n = 5) perovskites. The in situ XRD measurements confirmed that γ-phase CsPbI 3 was released during the structural transformation. Moreover, temperature-dependent in situ XRD measurements were employed to examine the kinetic energy involved in the structural transformation from the n = 6 form to the n = 5 form. Specifically, an intermediate structure from n = 6 to n = 5 was also identified. Most importantly, 2D Cs 6 Pb 5 I 16 ( n = 5) was more structurally thermodynamically stable than 2D Cs 7 Pb 6 I 19 ( n = 6). This study provides an essential route for the discovery of new types of perovskite structures during structural transformation.